MORPHOLOGY OF EYE MUSCLE» NERVES 131 



In objection to Dohrn's conclusions Belogolowy ('10 b, p. 23) 

 advances the following considerations: 



Dohrn, on the basis of the plurality of the ganglionic clumps connected 

 with the trochlearis anlage and the subdivision of the nerve into single 

 fibers, inferred the plurality and polymerism of the trochlearis. The 

 discoveries of Dohrn appear to me quite insufficient to draw such a 

 conclusion from, since it is impossible to use as a basis for such inferences 

 the plurality of position and number of such indefinite elements. Were 

 we to follow this method, it would be easy as the final result to con- 

 sider the maximum number of branches which might be met by chance 

 as the number of segmental nerves which have participated in the 

 formation of the trochlearis; and the presence of clumps of neural 

 crest cells which accompany these branches might be considered as suffi- 

 cient proof of our assumption. But it appears to me at least risky to 

 admit as a decisive criterion for our inferences the accidental occurrence 

 of this or that number of anastomoses or ganglionic clumps. 



On the other hand, there appear good reasons for regarding 

 the abducens as a nerve representing more than a single meta- 

 mere. In the first place its roots arise from at least two hind- 

 brain neuromeres (neuromeres VII and VIII) and its nidulus is 

 equally extensive; its transient and rudimentary ramus recurrens 

 is suggestive of an earlier distribution to posterior myotomes; and 

 lastly, it is distributed to myotomes other than those of its own 

 metamere. 



It might seem at first thought as if the most convincing evi- 

 dence bearing upon the question of the polymerism of the abdu- 

 cens and the other eye-muscle nerves is the simple fact that they 

 are distributed to three successive myotomes, Van Wijhe's first, 

 second and third. And were this the only relation to be taken 

 into consideration, such a conclusion would seem unavoidable. 

 This conclusion however conflicts with the evidence that at least 

 three neuromeres separate the niduli of the trochlear and abdu- 

 cens; and, further, the evidence that the abducens innervates 

 muscles of other metameres than those of the post -otic meta- 

 mere to which it belongs. Therefore the problem of the number 

 of metameres represented by the three eye-muscle nerves cannot 

 be solved by ignoring the neuromeric relations. 



